The present invention concerns a laser-writable molding material containing at least one polymer material or at least one precursor compound which can be polymerised to give a polymer material, and a particulate salt-like compound or a mixture of particulate salt-like compounds, which under the influence of laser light changes its color or leads to a color change in the polymer material. The invention further concerns moldings produced from such laser-writable molding materials.
Research has been concerned for a prolonged period of time with the development of laser-writable plastic materials as laser light can be focused to form very fine beams, which there is a wish to use for producing text and image on plastic articles. Most plastic materials as such however are not writable as they do not absorb laser light. Therefore, particulate light-sensitive compounds referred to as pigments are added to the plastic material, such compounds leading to a local change in color under the influence of laser light. That change in color can result from the fact that the pigment itself, with the absorption of laser light, changes its color due to chemical reaction, breakdown or physical conversion into a different modification, or the light energy absorbed by the pigment is transferred on to the adjacent plastic material and breaks it down or causes a color change in some other fashion.
There are many different pigments which have been described and investigated for that purpose in the past such as for example a very wide range of different metal oxides, metal phosphates, metal stannates and metal salts of organic anions.
EP-A-190 997 discloses a method of laser marking in which inter alia titanium oxide and antimony oxide are incorporated as pigments into a polymer material. EP-A-330 869 discloses the use of titanium oxide white pigment. EP-A-400 305 and EP-A-542 115 describe the use of copper hydroxide phosphate or molybdenum-(IV)-oxide as laser pigments. U.S. Pat. No. 5,063,137 discloses inter alia the use of water-free metal phosphates and phosphoric acid-based glasses. EP-A-797 511 describes the use of wafer pigments with a layer of doped tin oxide. U.S. Pat. No. 5,489,639 describes the use of selected copper salts (phosphates, sulfates, thiocyanates). EP-A-764 683 discloses the use of copper pyrophosphate hydrate and/or manganese sulfate hydrate as pigments. The use of boric acid anhydride is known from EP-A-808 866. WO-A-98/58805 discloses a series of copper phosphates as laser pigments. WO-A-99/55773 describes inter alia the use of zinc hydroxy stannate and tin (II)-oxalate. DE-A-199 05 358 discloses the use of alkali copper diphosphate. WO-A-01/00719 describes the use of antinomy oxide of particle sizes above 0.5 μm. EP-A-1 190 988 discloses the use of given bismuth mixed oxides. WO-A-01/78994 proposes the use of copper fumarate, copper maleate and mixtures thereof DE-A-100 53 639 discloses the use of selected salts, in particular various cobalt and iron phosphates. DE-A-100 34 472 describes the use of particles surface-modified with given silicon compounds. EP-A-753 536 describes the use of at least two kinds of metal oxides. EP-A-105 451 describes polyphenylene sulfide molding materials which for laser marking have been modified with selected additives, for example with the combination of lead chromate and lead molybdate, with nickel-antimony-titanate or with cobalt-zinc-silicon. DE 102004050481 discloses the use of tin phosphate as a pigment for laser writing. DE 102004050480 describes the use of a salt-like compound with at least two different metal cations as a laser pigment. DE 102004050571 describes the use of a salt-like compound or a mixture of salt-like compounds, which contains at least tin as a divalent cation and optionally further cations and oxoanions, anions of organic carboxylic acids or anions of carbonic acid.
Hitherto primarily thermoplastic materials have been used as plastic materials for laser writing such as polyethylene, polypropylene, polypropylene-polyethylene copolymers, polyamide, polyester, polyphenylene oxide, polyacetal, polyethylene terephthalate, polybutylene terephthalate, polymethacrylate, polyoxymethylene, polyvinyl acetal, polystyrene, acryl-butadiene-styrene, acryl-nitrile-styrene-acryl ester, polycarbonate, polyether sulfone, polyethylether ketone, polyvinyl chloride and thermoplastic polyurethane. Besides their advantages mechanical, thermal, electrical and chemical properties thermoplastic materials are also highly suitable for the incorporation of pigments for laser writing. Examples of use are uses in the domestic goods area, in relation to keyboards and in the electronic field, as described for example in EP-A-027 532.
In regard to the writability of thermosetting materials and elastomers by means of laser light, it is hitherto less known. Writing in respect of thermostable resins such as for example phenol resins, epoxy resins and other thermosetting materials has hitherto been effected with conventional printing methods such as ink jet printing, screen printing, tampon printing and so forth. The ink jet technology is usually employed when writing on silicone sheaths of cables. The conventional printing methods suffer from the disadvantage that the printing is not permanent and is easily removed by moisture, solvent or abrasion.